I just traded in an iPhone for a Samsung Captivate. Not sure about your carrier but I asked at AT&T if my SIM would work in both phones (thought it would be cool to switch it back and forth sometimes). They said no. I guess with AT&T the iPhone unlimited and the "Other" smartphone unlimited are two different plans and they are not switchable. At least thats what I was told by the salesperson. Haven't really tried it myself to see if its true.
Phone: SGS3 US AT&T Stock
Phone: Retired SGS Captivate SGH-I897 AT&T
ROM: AOKP Milestone 6
Tablet: WiFi Xoom ICS 4.0.4
ROM: Team Eos ICS 2.0.0 Stable